1. Field of the Invention
The present invention relates to a steering device with a constitution in which an operation unit operated by a driver and steerable wheels which are steered are not mechanically connected, and may be connectable from such a condition.
Priority is claimed on Japanese Patent Application No. 2004-114164, filed Apr. 8, 2004, the content of which is incorporated herein by reference.
2. Description of Related Art
In a so-called SBW (steer-by-wire) type steering device, there is no mechanical connection between the steering wheel (hereunder also referred to as the operation unit) steered by the driver, and the steerable wheels (the vehicle wheels). Therefore, greater output is required of the turning actuator which actually applies the turning angle to the steerable wheels, than in a conventional steering system in which the steering and the steerable wheels are linked mechanically, due to the fact that the steering effort of the driver applied to the steering wheel is not directly transmitted to the turning actuator.
Furthermore, in an SBW type steering device, the physical rack end position on the steerable wheels side cannot be felt at the steering wheel side by the driver. Therefore, it is necessary to make the driver aware of the so-called rack end bottoming, that is, the maximum turning angle.
One method of making the driver aware of the maximum turning angle is to rapidly build up the steering reaction force when the steering angle approaches the maximum steering angle, thereby letting the driver know when the turning angle reaches the maximum (for example, Patent Document 1: U.S. Pat. No. 5,247,441).
Incidentally, in the case of the SBW type steering device in which an electric motor is used in the turning actuator, there is a need for the output of the turning actuator to be as low as possible from the viewpoint of manufacturing costs and power consumption.
However, if the output of the motor is not sufficient, the driver may be unable to adequately turn the steerable wheels to the turning angle intended by the driver, that is the target turning angle determined according to the steering angle, so that the driver experiences a considerable feeling of unease. This feeling of unease is particularly marked at extremely low speeds where a large turning output is required, such as when parking, for example.
Furthermore, in Patent Document 1 mentioned above, if the steerable wheels do not adequately turn to the target turning angle, then during the time from when the steering wheel is turned to the maximum steering angle until the actual turning angle catches up to the maximum turning angle, the driver has to wait with the steering wheel kept at the maximum steering angle.
In order to solve this problem, it is also possible to prevent the driver from steering too quickly, by applying a steering reaction force corresponding to the deviation between the actual turning angle and the target turning angle (hereunder referred to as the turning angle deviation) to the steering wheel. However, if the turning actuator has insufficient motor output, there will always be a large turning angle deviation, and as a result, the driver's steering is constantly hampered by the large steering reaction force, and this lowers the quality of the product.